Seems to me that a single 15” driver would have a hard time reproducing the upper treble range.
And I agree with others that SET amps are quite capable of beautiful full range music reproduction with the right speakers. Devore as one example.
15 in full range drivers
I built a pair speakers with 15 in full range drivers that drive easily with a 300B set amp. With certain music, (vocals) they are beautiful, but other music can be fatiguing. Wife can only take them at lower volumes. Added great subs to support the base and the 300 B Set tubes, but I am grappling with just investing in a new pair of speakers that are more well rounded or trying to different DAC's and a Equalizer to help the speakers I have. I am thinking like one of the speakers from the Klipsch heritage line would be a great way to go. But getting rid of the ones I made is like putting down a dog that doesn't walk as far as he used to and fells like I am giving up on them. Do I go with new speakers or keep suffering? LOL.